After the letter was opened, three people at the residence were taken to New York Presbyterian-Weill Cornell Medical Center strictly as a precaution, the New York City Fire Department said.

There is no indication anyone suffered any injuries and no sign that the substance was a hazardous material or a biological agent.

.@NYPD tells @ABC Donald Trump Jr received a letter at his NYC home this morning containing white powder. His wife Vanessa opened it. 3 people were decontaminated & evaluated & appear to be fine. No positive identification on the powder yet; NYPD & Secret Service investigating.

Multiple senior law enforcement officials and city officials tell NBC News that a suspicious letter mailed to an apartment belonging to Vanessa Trump and addressed to Donald Trump Jr. was opened this morning and is being investigated by police.